This invention is directed toward generation of electromagnetic waves using electron beams and particularly to electron emission from a cold electrode.
The wavelength of electromagnetic radiation depends on the energy changes associated with the charges, atoms and molecules whose motion is responsible for the generation of the wave. Various techniques are employed to stimulate these energy changes and the parameters of the motion, hence the wavelength, determines the frequency of the resultant radiation.
Waves of broadcast frequencies are generated by current flow in an antenna. Infra red waves are generated by applying heat to a body whose molecules then change their energy state to radiate the infrared waves. X-rays are generated by directing a high energy electron beam against a target.
There has been disclosed x-rays generated by electron beams emitted by a cold cathode. In the early versions of this technique, the electrons ere emitted from the tips of needles or xe2x80x9cknife edgesxe2x80x9d by the very intense fields generated at the tip or blade edge.
For example, U.S. Pat. No. 3,714,486 to McCrary disclosed an electron beam originating from the tip of a needle.
U.S. Pat. No. 3,735,187 to Rogers disclosed a technique for making a very fine tungsten blade wherein an electron beam was generated along the sharp edge of the blade.
U.S. Pat. No. 3,883,760 to Cunningham discloses a field emission x-ray rube having a graphite fabric cathode in which the cathode comprises a concentric array of carbon fabric disks, each having a central aperture and with a needle anode located along the axis of the disks.
U.S. Pat. No. 4,958,365 to Sohval et al discloses a medical imaging device using a plasma cathode flash x-ray source.
U.S. Pat. No. 5,068,884 to Choe et al discloses an x-ray generation system for ultrafine lithography including a center electrode and a peripheral electrode with gas flow holes for generating x-rays from plasma.
U.S. Pat. No. 5,014,289 to Rothe discloses a cold cathode corona discharge to which is applied an accelerating potential forming an electron beam directed at a target to generate x-rays.
U.S. Pat. No. 5,469,490 to Golden et al discloses a cold-cathode x-ray emitter and tube powered by a light weight battery in which all of the high voltage components are contained in a sealed container.
U.S. Pat. No. 5,473,218 to Moyer discloses a flat, cold-cathode electron emitter including a substrate having a flat surface with a low work function electron emission material layer for emitting electrons supported on a substrate, a contact conductive layer with an aperture and an insulating layer, having an aperture coextensive with the first aperture. A conductive gate layer is disposed on the insulating layer. The conductive layer forms the field potential so that emission occurs substantially from the center of the aperture.
U.S. Pat. No. 5,604,401 to Makashima discloses a field emission cold cathode comprising an emitter support structure of two sets of fingers, the fingers of one set interleaving with, coplanar with, but not in electrical contact with the other set. Each set of fingers support a plurality of pointed cones. The base of the cones is supported on the respective array of fingers. Electrons are emitted from the tips of the cones, pointing away from the plane of the array of fingers. The device is used to operate in two current modes in a microwave tube.
X-ray tubes of the type having emission from the tip of a needle or blade are characterized by deterioration of the point or edge due to the intense field at these locations. This construction is useful only in applications where it is desirous to produce narrow beams.
This problem is avoided to some extent by those x-ray sources that use plasma discharge but, again there are limitations imposed by the geometrical arrangement of the tube components in attempting to provide a uniform beam having a broad cross sectional area.
The same limitation applies to tubes that rely on arrays of small apertures. The area around the apertures erodes due to the strong localized fields required.
FIGS. 1A and 1B illustrate a type of x-ray tube construction of the prior art having a housing 11 supporting a window 12 from which an x-ray beam 14 is emitted for purposes where a beam having a large cross sectional area is required. The prior art end window 12 of FIG. 1A consists of a Beryllium disk with the x-ray target material being the anode 16 bonded to the vacuum side of the beryllium disk 12. Electrons 18 are emitted from the hot cathode filament 20 and are accelerated by the high potential between the window 12 and filament 20. X-ray radiation 14 is generated by the electrons 18 striking the target and is transmitted through the target and window assembly.
The x-ray tube windows 12 of FIG. 1 must always be very thin (less than 250 microns) or too much energy is will be absorbed by the window material. Because the windows are very thin they can not tolerate much energy from the electron beam. Accordingly, beam power is limited to five watts or less.
As shown in FIG. 1B, in order to accommodate greater beam power, a separate anode 16 and window 12 are used which greatly increases complexity and cost of the device (x-ray tube).
In the prior art of FIG. 1A, the target layer thickness 16 must be accurately controlled to generate the required x-ray output. If it is too thick, the x-ray energy will be reduced by attenuation in the target material. If the target layer thickness is too thin, electrons will penetrate through the target and generate x-rays from the beryllium window 12 which contaminates the x-ray spectrum. Precisely controlling the penetration of the target 16 also limits the operation of the x-ray tube to a very narrow voltage range. Thus, multiple tubes must be used to provide the coverage of more than a very narrow band within the X-ray spectrum.
It is an object of this invention to provide an electron emitter integrated with a window which serves as the port through which electromagnetic energy enters or leaves the device.
It is another object that the cross section and intensity of the electron beam be controlled by the electric field at the emitter surface.
It is another object to provide a device for emitting radiation in a broad range of selected frequencies.
It is another object to emit a beam having a large intensity.
It is another object to eliminate the requirement to heat an emitter.
It is another object to provide a device that has a simple construction and is therefore economical to build
It is another object to build a source of electromagnetic radiation that has a broad application in many devices.
This invention is directed to structures including a sheet of electron emitting material that is integral to the window and emits electrons in the interior of the device. The electrons are accelerated to an appropriate energy level by a voltage applied to internal electrodes of the device and interact with an internal target structure to produce electromagnetic radiation or perform other useful functions. The radiation passes back through the cathode sheet and is emitted from the side of the cathode sheet opposite the target. The wavelength of the radiation emitted from the target through the window of the device depends upon the applied voltage between the emitter and the anode as well as the nature of the anode.
In the context of this specification, the term xe2x80x9ctargetxe2x80x9d or xe2x80x9ctarget structurexe2x80x9d is interpreted to mean the component of the device that interacts with the electron stream to generate electromagnetic radiation or other useful output signals of the device. The specific target structure that is selected depends upon the wavelength of the electromagnetic radiation or other useful signals that are to be generated.
In the radio frequency and microwave bands, the target has the form of a circuit propagating electric magnetic energy which is generated by means of interaction of the circuit with the emitted electrons. The propagating circuits may comprise one or more structures including resonators, TWT, EBS, a parametric amplifier, backward wave devices or diode arrays with dimensions appropriate to the frequency of the energy to be exacted.
In the infrared, visible light, and ultra violet ranges, the target may be a solid state laser that is pumped by the emitted electrons. Such targets would comprise materials such as Neodimium, YAG, InGaN/GaN, AlGaN/GaAs, Al G/InP.
To provide radiation at x-ray wavelengths, the target may be a simple metal sheet affixed to or comprising the anode. The wavelength of the radiation emitted from the target through the window of the device depends not only upon the applied voltage between the emitter and the anode but also on the metal which forms the surface of the anode.
X-ray tubes built according to the present invention do not have a limitation on target thickness because the electron beam is incident on the same surface of the anode as the wave emitting surface. Accordingly, the anode can be designed to accommodate electron beams of several hundred watts.
Since the electrons originate from the inside surface of the emitting window and the radiation is generated in a very thick target mounted on the anode, there is no limitation on the range of operating voltage. Therefore, the emitting window of this invention can be applied to generating a broad range of the electromagnetic spectrum extending from x-rays to microwave frequencies.
The advantages of the invention over the prior art reside in the fact that the electron emitter and input and output ports from the device are a single component without any separate electrical connections as opposed to heated cathodes or cold emitter arrays which require additional electrodes, electrical feedthroughs and additional voltages supplies for operation. Simplification of the device and elimination of power supplies results in lower manufacturing cost, reduced power consumption and, in some cases, increase the frequency bandwidth and power output that the devices can provide.
Materials are much more transmissive for electromagnetic waves than for an electron beam. This property provides that, for some applications, the cathode sheet may be thick enough to be self supporting. When an emitting window is required to be large so that so that greater structural support is required, it is an embodiment of the invention to provide a cathode/window that is a composite of two layers. One layer, closest to the target, is made of one material selected because of its ability to emit electrons and is thin enough not to block the electromagnetic wave. This sheet is supported by a thicker sheet selected for its ability to transmit the generated radiation.
In another embodiment of the laminate structure, where both the electron emitting layer and the thicker support layer are poor electrical conductors, a third thin layer of a god electrical conducting material is interposed between the support layer and the electron emitting layer to which electrical contact is made for applying the accelerating potential between the anode and electron emitting cathode.